In cytodiagnostics and pathology, for example, stained specimens are examined under the microscope, usually with transmitted-light bright field illumination. The colour of the specimen examined under the microscope is an important criterion for the diagnosis.
In recent decades, halogen lamps have been used as lighting means in the microscope, e.g. for transmitted-light bright field illumination. The light emitted by the halogen lamp corresponds primarily to the continuous spectrum of a black body radiator (Planckian radiator). Usually, a thermal protection filter that greatly attenuates the infrared range of the radiation emitted is incorporated in a lamp housing with a halogen lamp. Often an absorbent glass (KG1, 2 mm thick) is used as the thermal protection filter. The continuous spectrum of the resulting illumination enables the user to arrive at a reliable assessment of the stain colour.
For evaluating colours under illumination with a specific light source, the so-called Colour Rendering Index, CRI, is of importance. By this is meant a photometric value which can be used to describe the quality of colour reproduction of light sources of the same correlated colour temperature. The reference used for assessing the colour rendering quality, up to a colour temperature of 5,000K, is the light emitted by a black body radiator of the corresponding colour temperature. Beyond a colour temperature of 5,000 K, a spectral distribution similar to daylight is used as reference. For example, the spectrum of a black body radiator with a temperature of 2,700 K is used to calculate the colour rendering of a domestic incandescent lamp, which is itself a close approximation of a Planckian radiator. Each light source that perfectly replicates the spectrum of a black-body radiator of the same (correlated) colour temperature in the range of the visible wavelengths achieves a colour rendering index of 100. Halogen lamps, like incandescent bulbs, may achieve colour rendering indices of up to 100.
In microscopy, the halogen lamp is increasingly being replaced by light-emitting diodes (hereinafter LEDs) with their known advantages. These advantages include a higher radiation of light with a lower consumption of electric power and a longer life. White light LEDs are predominantly used for transmitted-light illumination. In a standard commercial white-light standard LED, a blue, violet or a UV-LED is combined with photoluminescent material in a housing as an integrated bluish unit. Usually, a blue LED is used which is combined with a yellow fluorescent substance. UV-LEDs with a number of different fluorescent substances (usually red, green and blue) may also be used. According to the principles of additive colour mixing, white light is produced with LEDs of this kind. The components thus produced have good colour rendering properties, with the colour rendering indices being between 70 and 90. However, white light LEDs do not emit a continuous spectrum. The white light LEDs based on blue LEDs have strong emissions for example in the blue spectral range (at about 450 nm), a minimum in the blue-green (at about 500 nm) and a stronger emission range at higher wavelengths with a maximum at about 550 nm, which drops away sharply at about 650 nm.
The ratio of intensity minimum at 500 nm to intensity maximum at about 450 nm is approximately 10-20%, depending on the type of LED. With a non-continuous spectrum of this kind as the specimen lighting, colour assessment is more difficult and deviates from the experiential values obtained by microscope illumination using a halogen lamp.
DE 10 2007 022 666 A1 addresses this problem. To provide a continuous spectral distribution that corresponds as closely as possible to daylight, this publication proposes adjusting the ranges of the above-mentioned intensity maxima to a lower and substantially identical intensity, using filters. In this way a continuous spectrum can be simulated, although it corresponds only somewhat to that of daylight. Another disadvantage is the loss of intensity that accompanies the use of such filters. For this reason, the white light LEDs used have to be very powerful, leading to higher energy costs, the generation of more heat in the microscope and a bulkier construction (on account of the cooling means or fans required).
Consequently, the problem of the present invention is to provide powerful illumination for a microscope by means of which as continuous a spectrum as possible can be effectively produced, corresponding substantially to the daylight spectrum or perceived as such by an observer.